Bipolar separator for fuel cell stack

ABSTRACT

It is described a bipolar separator for polymer membrane fuel cell stacks, delimited by two sheets provided with fluid passage holes connected by means of a corrugated element and comprising a passage section for a thermostatting liquid, which allows to achieve the withdrawal of heat from the adjacent cells and the humidification and distribution of gases with a single integrated piece, simplifying the assembly and the hydraulic sealing of the stack.

This application is a 371 of PCT/EP2004/010930 filed Sep. 30, 2004.

DESCRIPTION OF THE INVENTION

The present invention relates to a bipolar separation element betweenfuel cells, in particular polymer membrane fuel cells, laminated in astack in a filter press configuration.

As is known in the art, fuel cells are electrochemical generatorsconverting the chemical energy of the reaction between a fuel and anoxidant to electrical energy, producing water as a by-product. Among thevarious known types of fuel cells, the polymer membrane type is the onewhich operates at the lowest temperature, typically 70-100° C.,providing sensible advantages in terms of easiness and safety ofoperation, of material stability and especially of quickness in start-upand in reaching the final regime operating conditions. Among the mainproblems which have slowed the industrial diffusion of this technology,one of the most significant lies in the fact the energy produced by asingle cell is obtained as direct current of relatively high intensityversus a very limited voltage (in any case lower than 1 V, and typicallycomprised between 0.5 and 0.8 V). This characteristic, whose reasons areof thermodynamic nature and thus intrinsic to the process, makesnecessary the lamination of a certain number of cells in stacksassembled in accordance to a filter-press type arrangement. The stacksproposed for an industrial utilisation consist therefore of some tens ofelements, not seldom exceeding one hundred single cells; this entails,besides the apparent problems associated with the constructivetolerances and with the tightening of the final module, also assemblingtimes heavily affecting the final cost, each cell consisting of amultiplicity of pieces, including bipolar plates, gaskets, currentcollectors and electrochemical components such as electrodes andmembranes.

The constructive complexity of polymer membrane fuel cells is imposed bythe multiplicity of functions required to make the reactions of fueloxidation and oxidant reduction proceed with a high efficiency. Besidesthe optimum functioning of the electrodes which are the sites of the tworeactions, and which must be provided with adequate catalysts, generallybased on noble metals, a critical factor is given by the ion-exchangemembrane acting as the solid electrolyte and which must provide fortransporting the electrical current as a flow of ions; in particular,the protons generated by the oxidation of the fuel, that in the mostcommon of cases consists of hydrogen, either pure or in admixture, haveto cross the membrane thickness and be transported to the cathode sidewhere they are consumed by the reaction with the oxidant, generallyconsisting of oxygen, also pure or in admixture. The ion-exchangemembranes currently available on the market consist of a polymericbackbone, often perfluorinated for the sake of chemical stability,whereto anionic functional groups are attached, capable of bondingprotons albeit to a sufficiently weak extent to allow the migrationthereof under the effect of the electric field generated by thereactants. In order for this mechanism to be effective, in other wordsin order for the membrane ionic conductivity to be sufficient, it isnecessary to maintain a high degree of hydration of the membrane duringoperation. For most of the operating conditions of practical interest,the water produced at the cathode by the reaction of oxygen with theprotons coming from the anode side is not sufficient to guarantee thatsuch hydration conditions are always maintained; the flow of gaseousreactants supplied to the cells tends in fact to favour a consistentevaporation, which must be somehow counterbalanced. Maintaining anadequate water balance furthermore implies an accurate thermal controlof the cell, which constitutes another problem of no trivial solution.In conditions of electric power generation of practical use, the systemirreversibilities generate in fact a much relevant amount of heat, whichhas to be effectively withdrawn from the cells.

For the above stated reasons, polymer membrane fuel cells must beprovided with adequate devices for the humidification of the gaseousreactants and for the withdrawal of the generated heat. This isevidently in contradiction with the demand, prescribed by the market,for the availability of more and more compact systems characterised by aquick and easy assembling.

Whereas the first membrane electrochemical generators of the prior artwere constructed with components of ribbed graphite subjected to furthercumbersome machining, the most recent technological solutions providethe use of metallic materials with reduced thickness and more favourablemechanical characteristics. They are for instance configured asdescribed in U.S. Pat. No. 5,578,388, providing the supply of previouslyhumidified reactants to the two compartments, anodic and cathodic, of astack of cells delimited by preferably metallic bipolar plates, coupledto frame-shaped planar gaskets suitable for housing an adequate currentcollector also acting as distributing chamber, besides ensuring theelectrical continuity between the plate itself and the so-calledelectrochemical package; the latter consists of an ion-exchangemembrane-gas diffusion electrode assembly. The current collector is ametallic reticulated element, which favours the delocalisation of theelectrical contact and the distribution of the correspondent gas flowalong the whole surface of the membrane-electrode assembly. The heatwithdrawal is typically achieved through the circulation of water orother thermostatting fluid inside a serpentine embedded within thethickness of the metallic plate; this nevertheless entails the use ofrather thick and heavy plates, expensive to manufacture since they areobtained by a delicate moulding operation. As an alternative, stackconfigurations alternating, within the same lamination, fuel cells tothermostatting cells crossed by water or other cooling fluid capable ofexchanging heat through the walls of the metallic plates delimiting thevarious cells have been proposed. In this way, much thinner plates maybe employed and moderate weight reductions of the structures can beobtained, especially important for mobile applications, for instance forfuel cells destined to electrical vehicle transportation. On the otherhand, this solution does not offer a substantial improvement in terms ofsize, since the thickness reduction of the plates is obviouslycompensated by the addition of the thermostatting cells to thefilter-press structure.

Several cell designs have thus been proposed directed to decrease theweight and compact the fuel cell stacks integrating the differentfunctions in the best way and minimising the unemployed volumes: forinstance the co-pending international application PCT/EP 03/01207provides exploiting the peripheral part of the thermostatting cells fordistributing the gaseous reactants to the fuel cells, by means of aseries of openings obtained on the separating plate outside the zone ofcirculation of the cooling fluid.

For water-cooled cells, a still more advanced design, described in theco-pending international application PCT/EP03/06327, provides anexchange of matter, through appropriate calibrated holes, also insidethe cooling region; in other words, part of the cooling water is allowedto penetrate inside the fuel cells, performing the gas humidification insitu while carrying out an even more effective cooling because of apartial evaporation within the fuel cells. Besides enhancing the heatwithdrawal efficiency, this remarkably simplifies the overall system,allowing the elimination of the external humidification units;nevertheless, the two latter disclosed embodiments are rather complexunder the standpoint of hydraulic sealing. One of the main problems inthe manufacturing of filter-press structures with many laminatedelements consists in fact of the coupling of a high number of elasticgaskets, which must be compressed in a uniform fashion once subjected tothe tightening load, in order not to jeopardise the alignment of therigid components (and indirectly the electrical contact), while ensuringthe sealing of the different fluids, among which some are particularlycritical such as hydrogen. Notwithstanding the consistent improvementsin the gasket design and materials, it is very important to minimisetheir number in order to increase the reliability of the relevantsystems. The findings disclosed in the international applications PCT/EP03/01207 and PCT/EP03/06327 conversely present the evident drawback of aconsistent amount of gas-liquid and gas-gas seals, for instance twicethe amount of the invention of U.S. Pat. No. 5,578,388. Anotherdisadvantage intrinsic to this types of design, and in general to anydesign providing the alternation of fuel cells and thermostatting cells,is given by the complexity of the assembly, which provides laminating aremarkable number of components, which must be accurately disposed andperfectly centred, in a fixed sequence.

It is an object of the present invention to provide a fuel cell stackdesign overcoming the limitations of the prior art.

It is a second object of the present invention to provide a fuel cellstack design of high efficiency comprising a minimal amount of laminatedcomponents and of relative hydraulic seals for a given amount ofinstalled cells.

It is a further object of the present invention to provide an integratedseparator for fuel cells simultaneously achieving the internalcirculation of a cooling fluid, the distribution of the gaseousreactants to the cells and optionally the humidification of the latteror of just one of them.

Under a first aspect, the invention consists of a bipolar separatordelimited by a cathode sheet and an anode sheet, at least one of whichprovided with fluid passage holes, wherein said sheets are welded ormetallurgically bonded through a conductive corrugated element so as todelimit a cooling fluid passage section.

Under a second aspect, the invention consists of a stack of fuel cellsdisposed in a filter-press arrangement and separated by an integratedconductive element performing, in the different embodiments, one or morefunctions among which the thermal regulation of the cell, thedistribution and the humidification of the reactants without resortingto additional thermostatting cells.

The separator of the invention is delimited by two conductive sheets, atleast one of which is provided with fluid passage holes, respectivelysuitable for acting as cathode and anode sheet in a filter-press typebipolar arrangement. The two conductive sheets are mutually welded orotherwise secured through an interposed conductive element, whosegeometry is of the corrugated type in order to determine, in a preferredembodiment, the formation of channels for the passage of athermostatting fluid, preferably water in the liquid state. Bycorrugated element in this context it is intended a generic element, forinstance obtained from a planar sheet, with an undulated or otherwiseshaped profile so as to form projections and depressions; saidprojections and depressions are welded or otherwise securedalternatively to one or the other sheet delimiting the separator. Thecorrugated element has the dual purpose of mechanically adjoining theanode and cathode sheets and of ensuring the electrical continuitybetween the same. The corrugated element may be present just on aperipheral part of the separator, for instance in correspondence of twoopposed sides, or it may be disposed along the whole surface of thesheets. In the latter case, the corrugated conductive elementadvantageously delimits channels which can be used for the circulationof a cooling fluid, preferably liquid water. In case the corrugatedelement is present just in a peripheral region of the separator, usuallyoutside the cell's active area, the internal part may be advantageouslyfilled with a reticulated material suitable for being employed for thecirculation of a cooling fluid. As the reticulated material, metallicfoams or meshes, expanded sheets, sintered porous materials may beadvantageously used, also in mutual combination or juxtaposition;however, other types of reticulated materials may be employed withoutdeparting from the scope of the invention.

As said above, one or both of the sheets delimiting the separator areprovided with fluid passage holes; by fluid passage hole in this contextit is intended a through opening of any shape or profile, obtained onthe main surface of the corresponding sheet, suitable for being crossedby a liquid or a gas. In a particularly preferred embodiment, both ofthe sheets are provided with holes, preferably disposed along aperipheral region, in communication with a gas feeding duct; such holescan thus be employed to supply a gaseous reactant to the adjacent fuelcell, in a similar way as disclosed in PCT/EP 03/01207. Equivalentholes, in communication with a discharge duct, are preferably used fordischarging exhaust reactants and reaction products.

In a preferred embodiment, fluid passage holes, preferably in the formof calibrated orifices, are present in the internal part of the mainsurface of the separator, in correspondence of the cooling fluid passagesection. This embodiment is particularly advantageous, especially incase the cooling fluid is liquid water, since the controlled passage ofa portion of said cooling water from the inside of the separator to theoutside, toward one or both the adjacent fuel cells, determines thehumidification of one or both reactants, moreover contributing to theheat withdrawal by evaporation, in a similar manner as described inPCT/EP03/06327. The present invention thus exhibits the sameadvantageous features of the findings of PCT/EP 03/01207 andPCT/EP03/06327, making use however of an integrated separator directlyinterposed between the fuel cells, which replaces the thermostattingcells and the relative components to be individually assembled,simplifying the hydraulic sealing system by eliminating the relativegaskets and facilitating the assembly procedure to a radical extent.

For the sake of further favouring a quick assembly, and an error-proofone in the component alignment, the separator of the invention may bealso provided externally with current collectors and/or gaskets, weldedor otherwise secured on one or preferably both of the cathode and anodesheets. In such a way, the assembly of a stack would be accomplishedwith the minimum possible number of pieces, in the most extreme of caseswith just the separator provided with integrated collector and gasketbesides the electrochemical package consisting of an activated membraneor a membrane-electrode assembly as known in the art. Some of thepreferred embodiments will be now disclosed making reference to theattached figures, which have a merely exemplifying purpose and do notwish to constitute a limitation of the invention.

FIG. 1 shows a fuel cell stack according to the prior art.

FIG. 2 shows two embodiments of the separator of the invention.

FIG. 3 shows two other embodiments of the separator of the invention,comprising integrated gaskets and current collectors.

The fuel cell stack of FIG. 1 is configured in accordance with the mostwidespread teaching of the prior art, and comprises a juxtaposition oflaminated single fuel cells (100), delimited by separators (1) in formof bipolar sheets, which enclose an electrochemical package (2)consisting of an ion-exchange membrane activated on the two faces with acatalyst or by an ion-exchange membrane/gas diffusion electrodeassembly, as known in the art. The electrochemical package (2) dividesthe cell into two compartments, cathodic and anodic. The electricalcontinuity between the separators (1) and the electrochemical package(2) is ensured by the interposition of an appropriate current collector(3), which in the illustrated case is for instance a reticulatedconductive material also acting as a gas distributor. The hydraulicsealing of the cells is ensured by suitable gaskets (4), usually planegaskets. Each of the cells (100) is fed with a gaseous reactant, fueland oxidant, in the respective anodic and cathodic compartments, bymeans of suitable ducts not shown in the figure, as known in the art offilter-press type module design. The discharge of the exhausts and ofthe reaction products is likewise carried out by means of a collectingduct. A design of this kind does not provide the integratedhumidification of the reactants, which must be carried out externally,while the cell thermal regulation is typically carried out withserpentines, also not shown, embedded in the sheets acting as separators(1). Alternatively, thermostatting cells could have been intercalated tothe fuel cells (100), delimited by the same separators (1) andinternally crossed by a liquid coolant; in this case, the assembly andthe hydraulic sealing would have obviously been complicated by theaddition of the specified components.

FIG. 2 shows two possible embodiments of the separator (1) of theinvention; in both cases, the separator is delimited by sheets (5), onecathodic and one anodic, joined by means of a corrugated element (8)secured by weld spots (6, 9) or other forms of metallurgical bonding; inthe case illustrated on the left hand side of the figure, the corrugatedelement (8) joins the cathode and anode sheets (5) along the wholesurface delimiting a serpentine channel which may be advantageouslycrossed by a cooling fluid supplied from an appropriately connectedcircuit, not shown. In the case illustrated on the right hand side ofthe figure, the corrugated element is present only on a peripheral partof the separator (1), typically outside the cell's active area, whilewithin the recess delimited by the two sheets (5) in the internal part,a reticulated element (10) is present, which can be crossed by a coolingfluid supplied from an appropriately connected circuit, not shown. Inboth of the illustrated embodiments, the separator is therefore capableof providing for the thermal regulation of the adjacent fuel cells.Furthermore, in both variants are present, in correspondence of aperipheral region of the separator (1), suitable holes (7) which can beemployed for feeding gaseous reactants coming from gas feed ducts, notshown, in communication with said peripheral region, to the respectiveadjacent fuel cells. Likewise, the relevant holes (11) for the dischargeof the exhausts and of the reaction products toward external dischargeducts, not shown, are present. In this way, the separator (1) of theinvention performs the function of gas distributor to the cells,allowing to obtain a compact design taking advantage of what wouldotherwise be a dead zone. The constitutive elements of the separators(1) in FIG. 1 are evidently not reported in scale; the feed (7) anddischarge holes (11), for example, are usually tiny, and have beenmagnified in the figure with respect to the typical situation in orderto explain their function with better clarity.

In the version illustrated at the right hand side, the communicationholes between the inside and the outside of the separator (1) alsocomprise calibrated orifices (7′) which serve to allow a controlledpassage of cooling water toward the adjacent fuel cells: in this case,the separator (1) performs also the function of humidifying thereactants of the adjacent cells; the heat withdrawal from said cells ismoreover incremented by the evaporation of part of the water passingthrough the orifices (7′) inside the same cells.

The different characteristics of the separators in FIG. 2 have beencombined in a casual fashion, and what illustrated does not constitute alimitation of the invention; for instance, the calibrated orifices (7′)for feeding water could have been coupled to a corrugated element (8)present along the whole surface as in the case of the drawing on theleft, and so on.

FIG. 3 shows two embodiments equivalent to those of FIG. 2, furthercomprising the integration of the current collectors (3) and of thegaskets (4) of the fuel cells (100). In this way, the amount ofcomponents to be laminated for the realisation of the filter-pressconfiguration is reduced to a minimum. The current collectors (3) may beintegrated to the separator (1) of the invention by welding, also of thespot type, by soldering or other metallurgical bonding; the gaskets (4)may be integrated by moulding, gluing or by other systems known to thoseskilled in the art. Variations of the illustrated embodiments areevidently possible, without departing from the scope of the invention;for instance, the integrated bipolar separator (1) may comprise thecurrent collectors (3) and not the gaskets (4) or vice versa, or againit may comprise one or both of those elements on both sides or on oneside only.

As is apparent for one skilled in the art, the invention may bepractised making other variations or modifications to the citedexamples.

It must be intended therefore that the foregoing description does notwish to limit the invention, which may be employed according todifferent embodiments without departing from the scopes thereof, andwhose extent is univocally defined by the appended claims.

In the description and in the claims of the present application, theterm “comprise” and its variations such as “comprising” and “comprises”are not intended to exclude the presence of other elements or additionalcomponents.

1. A bipolar separator for a fuel cell stack, comprising: a cathodesheet and an anode sheet, at least one of said sheets provided withfluid passage holes; at least one corrugated conductive element, whereinsaid cathode sheet and said anode sheet are welded or metallurgicallybonded through said at least one corrugated conductive element; whereina cooling fluid passage is formed between the corrugated conductiveelement and at least one of said cathode sheet and anode sheet; andwherein said at least one corrugated conductive element adjoins saidcathode and anode sheets only in one or more peripheral regions of theseparator.
 2. The separator of claim 1, wherein said fluid passage holesare gas feed and/or discharge holes disposed in one or more peripheralregions of said at least one sheet.
 3. The separator of claim 1 whereinsaid fluid passage holes comprise calibrated orifices for feeding a flowof said cooling fluid into a fuel cell adjacent to the separator.
 4. Theseparator of claim 1 wherein a cooling fluid passage section comprisesat least one reticulated element interposed between said cathode sheetand said anode sheet.
 5. The separator of claim 4 wherein said at leastone reticulated element is an electrically conductive, optionallymetallic element.
 6. The separator of claim 5 wherein said at least oneconductive reticulated element is selected from the group consisting ofmetal foams, metal meshes, expanded sheets and sintered metallicmaterials.
 7. The separator of claim 1 wherein at least one of saidanode and cathode sheets comprises a sealing gasket secured to the sideopposite to the one whereto said corrugated conductive element is weldedor metallurgically bonded.
 8. The separator of claim 1 wherein at leastone of said anode and cathode sheets comprises a current collectorwelded or metallurgically bonded to the side opposite to the one wheretosaid corrugated conductive element is welded or metallurgically bonded.9. The separator of claim 8 wherein said current collector is anelectrically conductive reticulated element optionally selected from thegroup consisting of metal foams, metal meshes, expanded sheets andsintered metallic materials.
 10. A fuel cell stack comprising at leastone separator of claim
 1. 11. The stack of claim 10 comprising at leastone feed or discharge duct in communication with said fluid passageholes.
 12. The separator of claim 1, wherein the fluid passage holes insaid anode and/or said cathode sheet are gas feed and/or discharge holeslocated only in one or more peripheral regions.
 13. The separator ofclaim 1, further comprising a reticulated element interposed betweensaid anode sheet and said cathode sheet, forming a cooling fluid passagesection between said anode sheet and cathode sheet, wherein at least oneof said anode sheet and said cathode sheet has fluid passage holes in aregion in contact with the reticulated element.